1. Field of the Invention
The present invention relates to a variable-power type eyepiece lens system to be used in binoculars, telescopes and the like, and a variable-power type observational optical system which uses such a variable-power type eyepiece lens system.
2. Description of the Related Art
In a variable-power type observational optical system having an objective lens system and an eyepiece lens system in this order from the object, at least two types of lens systems have been known, that is, a type where variable power is performed by varying the focal length of the objective lens system, and another type where variable power is performed by varying the focal length of the eyepiece lens system. The above mentioned variable-power type eyepiece lens system is a type of a lens system in which an object image formed by an objective lens system is observed through the process of variable power. For example, U.S. Pat. No. 5,734,509 and Japanese Unexamined Patent Publication No.6-175048 have disclosed the above types of optical systems. These variable-power type eyepiece lens systems have five lens elements constituting three lens groups, and variable power is performed by moving the second and third lens groups, which are counted from the side of the eye-point, in the opposite directions with respect to the object image which is formed between the second and third lens groups. In these variable-power type eyepiece lens systems of the prior art, the eye-relief is relatively longer, and aberrations are satisfactorily corrected. However, the overall length of these optical systems are long, and the amount of change in the overall length of the optical system is large when variable power is performed. Moreover, in the embodiments of these optical systems disclosed in the above mentioned US patent and Japanese Unexamined Patent Publication, the five lens elements are provided to constitute three lens groups, and hence, the manufacturing costs of these optical systems are high.